Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
Jack Hogan
Veteran MemberPosts: 4,025
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Re: steephill, sjgcit, gl2k
In reply to JDinOregon, Nov 12, 2012

JDinOregon wrote:

By "Blind Exposure" I take the situation as similar to doing a portrait without the benifit of an incident light meter. The photographer does not want to guess at the reflectance of the subjects skin (although I remember something like one stop brighter than middle gray as typical for caucasions). So instead an 18% gray card is used as a standard.

Yes!

The meter constant for a Seconic Incident meter is 250 and the constant for Nikon and Canon reflected meters is 12.5. With exposure defined by

EV= log2(L*s/k) = log2(E*s/c)

This implies reflectance from a uniform perfect diffuser is 15.7%. From

L=E*R/pi

I think this means that the gray card will indicate an exposure about 0.2 stops off that of the incident meter. Not so much for the reason that 18% is the wrong reflectance, but rather that an average scene is not a flat plane. (L not equal to E*R/pi).

Hmm, interesting. But aside from different readings from different types of meters, aren't calibrated reflected light meters fully characterized by the constant K? And wouldn't this mean that if one were presented with a neutral card, irrespective of reflectance, it would produce values for aperture and shutter speed that when dialled into a DSLR at the same ISO should result in the following Exposure on the sensor

Hm = pi/4 * K/S

which with CaNikon's K=12.5 we could rewrite as about

Hm = 10 / S lux-seconds

which comfortingly is the same equation found in ISO 2240 and 2721 for the automatic controls of exposure in electronic imaging systems?

And that if the neutral card presented above were Middle Gray (18%, as calibrated) then the rest of the scene would be blind exposed 'properly'?

We know Hsat, because DxO handily measures it for us, so that leaves us with the pesky question of S ...

Jack

PS

It would be interesting to know what the model of an average scene was and also how that is interpreted by matrix metering in the different camera brands.

The following is not about matrix metering (a different beast altogether) but it may give you an idea:

Minolta IV users manual, page 3: " Standard reference subjects may have a reflection factor somewhere between 12% and 26%. Minolta bases its readings on a reference subject with an 18% reflection factor."

Sekonic Studio Deluxe L-398, page 2: " it is designed around a standard reflectivity of 18%, which has been derived through measurements of various subjects throughout the seasons of the year"

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